A theoretically-ideal bandpass filter passes any energy having a frequency within a desired frequency band and blocks any energy having a frequency outside the desired band. Similarly, an ideal notch filter blocks all energy having a frequency within a specified band and passes all other energy. The response curve of an ideal bandpass filter is shown in FIG. 1; the response curve of an ideal notch filter is shown in FIG. 2. In everyday terms, an example of an ideal bandpass filter would be a tuning circuit that would tune a radio to receive a desired station with perfect fidelity while totally rejecting all other stations, even a much stronger one in an adjacent frequency band. In the optical range, an ideal filter would allow, say, red light to pass at full brightness while totally blocking light of all other colors.
For many reasons, theoretically ideal filters cannot be achieved in practice. A good, practically realizable filter is generally considered to be a filter having a response that is essentially flat throughout the desired pass band and that smoothly drops off with either increasing or decreasing frequency outside the pass band. The response curve of a good realizable bandpass filter is shown in FIG. 3.
An acousto-optic tunable filter ("AOTF") is an electronically tunable optical bandpass filter. Bulk AOTFs--that is, AOTFs fabricated in bulk crystals and using bulk acoustic waves and unguided optical beams--have already found many important applications in laser and optics systems. Integrated AOTFs--AOTFs in which light is confined to a waveguide and which use surface acoustic waves--are also expected to find important applications in laser and fiber optics systems, especially such as are used in modern telecommunications applications. The construction and operation of an integrated AOTF will now be explained with reference to FIG. 4.
An integrated AOTF is fabricated in an elongated crystalline substrate 11 such as lithium niobate (LiNbO.sub.3). An optical waveguide 13 is formed in an upper surface of the substrate, for example by indiffusion of titanium. A beam of light is coupled into a first extremity 15 of the waveguide 13 through an input optical fiber 17. The light propagates through the waveguide and out through an output optical fiber 19. A surface acoustic wave is induced in the waveguide by an interdigitated transducer 21. The transducer is driven by an externally-generated electrical signal; the frequency of the acoustic wave is determined by the frequency of the electrical signal.
The acoustic wave induces a diffraction grating in the waveguide, and this in turn diffracts the beam of light. The grating couples the transverse electric and transverse magnetic polarization modes of the light, but only within a narrow band of optical wavelengths. Thus, within this narrow band all the light propagating in one polarization mode is converted to the orthogonal mode, whereas outside this band the polarization mode of the light is unaffected.
A TE pass polarizer 23 adjacent the first extremity of the waveguide blocks any incoming light that is not in a first polarization mode. Thus, only light polarized in the first mode is admitted to the filter. As the light travels through the waveguide, the polarization mode of any of the light having a wavelength within the aforesaid narrow band of optical wavelengths is converted to a second mode which is orthogonal to the first mode. The polarization of the rest of the light is unaffected. A TM pass polarizer 25 opposite the polarizer 23 blocks from the output any light that is not in the second polarization mode. Thus, only light having a polarization mode that has been converted while passing through the filter is allowed to exit the filter. No output destination is shown, but it will be understood that the output light is ultimately provided to a user or to an optical device of some type.
From the foregoing it will be seen that the AOTF passes light having a wavelength within the band determined by the acoustic wave and blocks other light. Thus the AOTF serves as a bandpass filter. The center frequency of the pass band can be tuned by changing the frequency of the electrical signal that drives the transducer. The filter may be converted into a "notch" filter by changing the polarizer 25 to the same type of polarizer as the polarizer 23
The polarizers 23 and 25 are shown as having been integrally formed in the substrate 11. However, one or both of these polarizers may be fabricated as separate devices and located remotely from the substrate 11 in the input and output light paths, respectively. Indeed, the polarizer 23 may be omitted entirely if the input light is already polarized, and the polarizer 25 may be omitted if the user or optical device that receives the output light is able to discriminate among polarization modes.
As the light passes through the waveguide and is diffracted by the acoustic wave, the frequency of the light is Doppler-shifted because the grating induced by the acoustic wave is in motion with respect to the waveguide. If desired, this Doppler shift can be cancelled by passing the light through a second AOTF. Although two physically separate AOTFs could be used, it is often advantageous to fabricate two AOTF stages in two adjacent sections 27 and 29 of the waveguide 13. In such a configuration the acoustic wave generated by the transducer 21 is confined to the first waveguide section 27 by acoustic absorbers 31 and 33. The acoustic absorbers may be fabricated of wax or other convenient materials. A second transducer 35 generates a second acoustic wave in the second waveguide section 29. This second acoustic wave is confined to the second waveguide section by acoustic absorbers 37 and 39.
A third polarizer 41, similar to the first polarizer 23, is located on the output end of the second waveguide section 29. If all three polarizers are the same type, a two-stage notch filter is formed. If the polarizer 25 passes only light that is orthogonally polarized to the light passed by the polarizers 23 and 41, a two-stage bandpass filter is formed.
Although a configuration with two transducers is shown, it may be preferable to use only one transducer to generate the acoustic waves in both sections of the waveguide. In such a configuration the acoustic absorbers 33 and 37 that lie between the two sections 27 and 29 are omitted. A single transducer generates the wave throughout both sections of the waveguide; the transducer may be located at any convenient point along the waveguide.
Unfortunately the frequency response of an AOTF is not like that of the good bandpass filter as shown in FIG. 3. Instead, the frequency response of an AOTF is characterized by unwanted sidelobes. Even in theory, the best performance that can be expected from a basic single-stage AOTF is sidelobes not more than ten decibels ("dB") below the center frequency, as explained by Harris et al., Journal of the Optical Society of America, vol. 59, page 744 (1969). In practice, single-stage AOTF sidelobes are often much less than ten dB below the center frequency. Even worse, the sidelobes are not symmetrical. A response curve of a typical AOTF, showing the asymmetric sidelobes, is provided in FIG. 5.
Various methods of reducing these unwanted sidelobes have been proposed, and by way of example some of these proposals are discussed in Kar-Roy et al., IEEE Photonics Technology Letters, vol. 4, page 1132 (1992); Smith et al., Integrated Photonics Research, Vol. 10 of 1992 OSA Technical Digest Series (Optical Society of America, Washington) pp 88-89; Herrmann et al., Electronics Letters Vol. 28, page 979 (1992); and Herrmann et al., Electronics Letters, Vol. 28, p. 642 (1992).
An explanation of why unsymmetric sidelobes occur, and a proposal for eliminating them, may be found in Trutna et al., "Anomalous Sidelobes and Birefringence Apodization in Acousto-Optic Tunable Filters", Optics Letters, Vol. 18, No. 1, Jan. 1, 1993. In brief, Trutna et al. showed that the undesired sidelobes result from birefringence nonuniformity in the waveguide. This nonuniformity in turn has several causes including thermal gradients, waveguide width variations, titanium thickness variations, and inhomogeneity in the LiNbO.sub.3 substrate. Trutna et al. also showed how these variations might be intentionally used to reduce sidelobes in a two-section AOTF.
Integrated AOTFs are expected to find a particularly important application in making tunable lasers. Although various methods of tuning certain kinds of lasers are known, all of these methods suffer from such drawbacks as slow response, mechanical complexity, drift over time, and relatively high cost. The integrated AOTF offers the potential of overcoming many of these drawbacks if its filter function characteristics could be improved.
In view of the foregoing, it will be seen that there is a need for a way to improve the filter function characteristics of AOTFs so that the advantages which in theory can be obtained from these devices may be realized.